The invention concerns a peak detector of a variable incoming signal and includes, a memory, a first comparator for comparing a representation of the instantaneous value of the variable incoming signal with a value stored in the memory and a control unit for changing the memorized value. Such a detector can be used anywhere when it is required to supervise the operating of a transmission channel, whether the signal be analog or digital. A particular application is found for the detector in the branch of a four-wire telephone transmission system, in which the state of a circuit must be known at each instant in order to be able to cancel the echoes or block the transmission of noises when the subscriber is not speaking, these operations, moreover, requiring, generally, a comparison between the level of the signals on the transmitting and receiving channels of a subscriber.
To know the level of a signal, an integrating of that signal is generally performed and a comparison of the ingegrated value with a fixed threshold enables the distinguishing of the useful signal from the noise. If the processing of an analog signal according to this principle does not create any major problem, it will be observed that the integration of a variable which is represented digitally leads to complicated and expensive devices as shown for example in U.S. Pat. No. 3,825,694 of issued July 23, 1974.
Another solution to the problem, which is better suited to the needs of a digital echo suppressor, is described in U.S. Pat. No. 3,896,273.
The integration is substituted, therein, by a detection of digital levels and a comparison of those levels with a threshold. Each threshold is valid during 50 ms time interval and all the values of the amplitude of the incident signal and stored during the original interval of 50 ms in memory cells. After the passing of those 50 ms, the threshold is replaced by the highest value which had been found in that period.
Such a device, although it is not as expensive as a real integrator, nevertheless requires a high-capacity-memory to store a great number of distinct values and comparators having a great number of inputs.
Furthermore, another circuit, in which the instantaneous value of an incident signal to be processed, in digital form, is compared with a variable threshold stored in a memory, is also known. When the instantaneous value of the incident signal is greater than the threshold, or when the threshold has not been modified during a predetermined period, the instantaneous value becomes the new threshold.
Such a circuit, which is very simple, does not alone, satisfactorily solve the integration problem in the case of an echo suppressor and another circuit, establishing, on the basis of the thresholds memorized successively and according to a predetermined law, values representing the envelope of the incident signal is required. Nevertheless, the combination of those two circuits, on the one hand, is fairly complex and on the other hand, forms a fairly slow system which can cause delays in the cancelling of echoes.